1) Field of Invention
The invention is in the general field of gas analysis. In particular it relates to the determination of the dew point of gases as well as detection and analysis of the components of gases.
2) Background
Dew point measurement of various compounds under different pressure regimes is an essential need in a number of applications. Since dew point corresponds to the vapor pressure of a given compounds at a given pressure, dew points are also routinely used as a measure of concentration of a material is gas phase.
The most common dew point measurement is used with water. The dew point of water is used in measured in many industrial applications. Both the raw dew point and the concentration inferred from the dew point are essential control parameters in many applications. For example water dew point, and the related partial vapor pressure, is needed in the semiconductor industry, the natural gas processing and transmission industry, air-dryer industry, and generally any process where the concentration of water needs to be measured.
However, dew point measurements are also needed and measured for many other compounds. For example in natural gas processing and transmission, hydrocarbon dew point of the gas stream needs to be determined for safety considerations as well as determining the value of the gas.
The most common method that has been used for many years for measuring the dew point is a chilled-mirror system. In this system, a mirror is chilled, and the surfaced is monitored to detect traces of condensation. The temperature at which condensation takes place is the dew point.
The detection was originally done by visual observation. This gave rise to errors emanating from human error and variability associated with operator skill.
More recently various means of automatic detection of the condensation have been described in prior art. One of the most commonly used methods involved shining a light beam, sometimes a laser beam, at the surface and monitoring for changes in the reflected beam as evidence of condensation on the surface (U.S. Pat. No. 5,028,143; U.S. Pat. No. 4,946,288; U.S. Pat. No. 4,799,235; U.S. Pat. No. 5,482,371). This method also suffers from a number of shortcomings. One of the shortcomings is that in the early stages of the condensation process, the changes in the reflected beam are not significant enough to indicate the exact dew point. The other shortcoming, encountered when there are various condensable vapors in the stream is that it is not clear what has been condensed on the surface first. For example, when the gas to be measured contains hydrocarbons and water at unknown quantities, either one may condense first depending on their relative vapor pressure and the total pressure and the temperature of the mirror. The changes in the reflected beam, typically cannot distinguish the substance that has been condensed. Another shortcoming is that these mirrors, typically made of metal, will be contaminated and degrade over times and their characteristics and thus their reflectance properties will change. A further problem is that the light beam actually has to travel through some of the gas before it hits the mirror and before it hits the detectors. This requirement makes the setup harder to implement.
There are other, non-optical methods described for detection of dew formation. Prior art described detection of dew formation by ultrasonic means (U.S. Pat. No. 6,327,890), MEM based devices (U.S. Pat. No. 6,126,311), measuring heat flow to the surface (U.S. Pat. No. 5,165,793), impedance changes (U.S. Pat. No. 4,948,263). None of these methods can determine the nature of the condensate accurately and also have reliability issues.